What Are Contrails? Why Some Planes Leave White Lines in the Sky and Others Do Not
Daily Explainer: one everyday bit of aviation, in plain English.
Look up on a clear day and you will often see a jet far overhead trailing a long white line. Some of these lines vanish within seconds. Others stretch across the sky for miles and slowly spread into a hazy veil. They are called contrails, and despite what some online rumours suggest, they are not smoke and not a secret spray. They are mostly just clouds made by the aircraft.
What a contrail is
"Contrail" is short for "condensation trail". It is a thin cloud of ice crystals that forms behind an aircraft flying at high altitude.
The ingredients are simple. Jet engines burn fuel, and one of the products of burning fuel is water vapour. The exhaust leaves the engine very hot and full of this vapour. At cruising altitude, the outside air is extremely cold, far below freezing. When the hot, moist exhaust mixes into that cold air, the water vapour cools quickly, condenses into tiny droplets and then freezes into ice crystals. That ice cloud is the contrail.
You can see the same thing on a cold morning when your breath forms a little white cloud. Your breath is warm and moist, the air is cold, and the vapour turns visible.
Why do some contrails vanish and others last?
This depends on the air the aircraft is flying through.
- If the surrounding air is dry, the ice crystals evaporate again quickly, and the contrail disappears within seconds or a few minutes. You may see only a short line right behind the plane.
- If the surrounding air is cold and humid, the ice crystals do not evaporate. They can grow, and the trail can last for many minutes or even hours. It may spread out sideways until it looks like thin, wispy cloud.
That is why you sometimes see one plane with a long trail and another, at a similar height nearby, with none. The two may be passing through slightly different layers of air, or flying at different altitudes.
Why do contrails only appear at height?
Lower down, the air is warmer, so the exhaust vapour does not freeze into ice in the same way. Contrails normally form at the altitudes where airliners cruise, where temperatures are far below freezing. At the low altitudes of take-off and landing you rarely see them.
What about those trails behind the wingtips?
Sometimes you may see short, brief streaks from the wings, especially at take-off or landing on a humid day. These come from a different effect. Air pressure drops sharply over a wing and in the swirling air at the wingtips, and in humid conditions, the moisture in that air can briefly condense into a visible cloud. It is a temporary effect of the airflow, not the same as an engine contrail.
Are contrails harmful?
Scientists study this. Contrails that spread out into thin cirrus-like clouds can trap heat that would otherwise escape to space, while also reflecting some sunlight. Researchers are looking at how large the overall effect on the climate may be and whether small changes in flight altitude or routing could reduce persistent contrails. Studies and trials are continuing, and the findings are still being refined, so it would be wrong to treat any single number as settled.
What is clear is that contrails are made mainly of water, in the form of ice, produced by ordinary jet engines.
Is it a "chemtrail"?
No. The idea that contrails contain secret chemicals or are deliberately sprayed has been looked into by atmospheric scientists and has not been supported by evidence. Contrails look different from day to day because the weather is different, not because the planes are doing something different. A long lasting trail simply means the air at that height was cold and humid.
What does it tell you about the weather?
Weather forecasters have long noted that contrails that persist and spread can be a sign that the upper air is humid. Short-lived contrails suggest dry air at that height. It is not a precise forecast tool, but it is a clue.
Quick answers
- Is it smoke? No. It is condensed water vapour and ice.
- Do all planes make contrails? No. It depends on the height, the temperature and the humidity.
- Do contrails mean a plane has a problem? No. They are a normal result of jet engines at altitude.
- Why do I see more on some days? Because the upper air is colder or more humid on those days.
The takeaway
A contrail is a thin ice cloud made by an aircraft's exhaust in very cold air. How long it lasts depends on how dry or humid the air is. Next time you see a long white line across the sky, you will know it is weather made by a plane.